In drilling oil or gas wells, mud-laden fluid or "drilling mud" is typically circulated through the borehole to cool and lubricate the drill bit, and to remove cuttings from the hole. The drilling mud is pumped into the hole from a nearby surface pond or pit, and is returned to the pit to deposit the various cuttings carried by the mud before recirculation through the hole.
The drilling mud provides a means of communicating with the bottom of the hole and with the geological formations penetrated by the drill bit. By appropriate measurements at the surface, before the mud is returned to the pit, useful data such as the concentration of oil, gas, water or sulfur in the drilling mud and cuttings, rate of drilling penetration, etc. can be determined through mud analysis. Such data can then be correlated with drilling depth to provide a log of useful information.
Examination of drilling mud and the cuttings carried thereby is known as mud logging. Although batch samples of drilling mud can be collected and analyzed, it is preferable to conduct sampling on a continuous basis. Continuous sampling is generally faster and more accurate than batch sampling. The control and amount of information afforded by continuous mud logging is particularly desirable when drilling exploratory holes, for instance, or when the stratigraphy is complicated.
While mud logging systems have been developed heretofore, the systems of the prior art have tended to be complex, expensive and difficult to operate. Most if not all of the prior systems for logging drilling mud on a continuous basis have required the attention of at least one operator. For example, one system of the prior art is truck-mounted and operator-attended, and consumes a large amount of power. The drilling mud is sampled inside the truck, which is usually parked a substantial distance away from the borehole due to the various drilling equipment which must be located around the hole. By the time the drilling mud reaches the truck, it may have become stale by percolation of gases out of solution, thereby reducing reliability and accuracy of the sampling. In addition, the expense of operating and maintaining such mud logging systems has made it infeasible to drill some exploratory holes that otherwise might have been drilled.
A need has thus arisen for a simplified mud logging system for analyzing the hydrocarbon content of drilling mud on a continuous basis with very little operator attention, except to activate the system and change the disk of a chart recorder at periodic intervals.